1) Field of the Invention
The present invention relates to a tandem type color image forming apparatus that detects toner density and controls image density based on the toner density detected.
2) Description of the Related Art
In recent years, there is increasing demand for achieving high productivity in image forming apparatuses that output color images according to the electrophotographic method. To respond to this demand, a tandem type image forming apparatuses have become developed. The tandem type image forming apparatuses include a plurality of image forming units, each having an image bearing member and a developer, are arranged in parallel so as to oppose to a transfer belt, and toner images on the image bearing members are sequentially transferred onto transfer paper (or transfer belt).
Japanese Patent Application Laid-Open No. 2000-206761 discloses an image forming apparatus that use a special technique. In this technique a reference toner patch is formed on an image bearing member or an intermediate transfer member, the toner density of the reference toner patch is detected by using an optical toner density sensor, and the image density is controlled based on the toner density detected.
However, the image forming apparatus, for example, the one disclosed in Japanese Patent Application Laid-Open No. 2000-206761, has problems as explained below.
Since the toner density sensors provided for each photoconductor are disposed at the same position with regard to the width direction of the photoconductors (width direction of the intermediate transfer belt) and one or more reference toner patches are formed in parallel for each color at the same position with regard to the width direction of each photoconductor so as to be detected by the respective toner sensors, the reference toner patches for each color, overlap on the intermediate transfer belt. As a result, unnecessarily large amount of toner is deposited on the intermediate transfer belt and that has to be cleaned by the intermediate transfer belt cleaner and the secondary transfer cleaner. Sometimes the intermediate transfer belt cleaner and the secondary transfer cleaner can not totally clean the toner because of its large quantity and this cause deterioration in the image quality.
One approach is to form toner patches for different color one after the other without overlapping the toner patches. However, this approach requires idle running. For example, if there are four colors, it takes almost four times longer time.
In other approach, toner density of reference toner patches is detected while shifting installing positions of toner sensors and imaging positions of the reference toner patches corresponding respective colors. However, since the intermediate transfer belt rubs against the transfer paper, and the transfer paper is easily damaged, the surface condition of the transfer belt is not uniform and the detection results obtained by the toner density sensors (for each color) disposed at different positions with regard to the width direction of the intermediate transfer belt significantly change. Therefore, detection stability is not ensured.
Also, when toner density changes in the image forming apparatus that detects toner density on an intermediate transfer belt, whether the cause of this change is toner density in the developer, potential of photoconductor or transfer rate of the intermediate transfer belt is unclear. For example, in the situation that the cause of decrease in toner density on the intermediate transfer belt is abnormal transfer rate to the intermediate transfer belt, if an attempt is made to increase the toner density by increasing the toner supply amount to the developer, the toner scattering may occur. Thus, detection of toner density on the intermediate transfer belt entails problems.